1. Field of the Invention
This invention relates to an optical pickup for recording and/or reproduction of an optical recording medium, a reproducing apparatus and a recording apparatus for an optical recording medium. More particularly, it relates to an apparatus capable of recording and/or reproducing plural sorts of optical discs different in track pitch by one and the same apparatus.
2. Description of the Related Art
Heretofore, in an apparatus for reproducing an optical disc, such as a compact disc, a three-beam method has used as a system for generating tracking error signals. This system splits a light beam radiated by a semiconductor laser element by a diffraction grating into three beams, namely a main beam and two side beams. The main beam is radiated on a recording track of an optical disc, while both side beams are radiated on the positions of the disc offset by one-fourth of a track to both sides of the recording track. The light beams radiated on the optical disc are reflected by the recording surface of the recording medium so as to be received by a photodetector. This photodetector is made up of a first photodetector portion for receiving both side beams. The tracking error signal is detected by finding the difference between the signals received and by the second and third photodetector portions.
Recently, an optical disc has been investigated in which data may be recorded to a high density for recording data of high precision, such as still pictures or moving pictures. With such an optical disc, it may be contemplated to set the track pitch to approximately 0.8 xcexcm instead of to 1.6 xcexcm conventionally used, or to form a recording layer of a narrower track pitch as multiple layers. The optical disc, recorded to a high density, is not limited to a replay-only type, but a rewritable optical disc, such as a phase change type disc, is also contemplated. Such a disc having a guide groove may also been envisaged as such rewritable optical disc.
However, it is difficult with an optical disc for high density recording to detect tracking error signals using the above-mentioned three-beam system. That is, since the track pitch is of a narrow width, registration of the side spots of the three spots radiated on the recording surface of the optical disc becomes difficult. Also, if the high-density recording layer is formed as multiple layers, a problem arises that an offset is produced in the tracking error signal due to leakage of the reflected light form the layer other than the layer being recorded or reproduced. In addition, if the rewritable optical disc is a phase change type disc, and recorded and unrecorded portions are produced on the disc, a noise is produced with the three-beam method due to difference in reflectance in the recorded and unrecorded portions, thus making it difficult to generate correct tracking error signals.
In view of the above-depicted status of the art, it is an object of the present invention to provide an apparatus capable of performing selective recording and/or reproduction on or from plural sorts of optical discs, such as optical discs with different track pitches.
An optical pickup according to the present invention includes a light source for radiating a light beam, a diffraction element for separating a light beam radiated from the light source into at least three beams, namely a main bean and two side beams, an objective lens for converging the light beams separated by the diffraction element on a signal recording surface of the optical recording medium, a light receiving unit having a four-segment first light receiving portion for receiving the main beam reflected by the recording surface of the optical recording medium and second and third light receiving portions arranged on both sides of the first light receiving portion for receiving the side beams reflected by the recording surface of the optical recording medium, and a calculation unit for generating a first tracking signal based on respective outputs of the first light receiving portion and for generating a second tracking signal based on outputs of the second and third light receiving portions.
A tracking servo system in an optical disc recording and/or reproducing apparatus according to the present invention includes a light source for radiating a light beam, a diffraction element for separating a light beam radiated from the light source into at least three beams, namely a main beam and two side beams, an objective lens for converging the light beams separated by the diffraction element on a signal recording surface of the optical recording medium, a light receiving unit having a four-segment first light receiving portion for receiving the main beam reflected by the recording surface of the optical recording medium and second and third light receiving portions arranged on both sides of the first light receiving portion for receiving the side beams reflected by the recording surface of the optical recording medium, a calculation unit for finding a plurality of tracking error signals based on outputs of the first, second and third light receiving portions, a discrimination unit for discriminating the sorts of the optical discs, a switching unit for selecting one of the tracking error signals calculated by the calculation unit based on a signal from the discrimination unit, and a driving unit for driving an objective lens based on the tracking error signal selected by the switching means.
The discrimination unit discriminates between at least two sorts of the optical discs with different track pitches, and the signal processing unit is responsive to the results of discrimination to switch the calculation operations for obtaining a tracking error signal from a detection signal of the photodetector unit, so that at least two sorts of the optical disc can be reproduced by simplified adjustment operation.